The present invention relates to composite materials, their method of preparation, as well as aqueous dispersions and paper-like sheets or webs used in said method, and to composite structures and articles made from the composite materials.
The term "composite material" as used herein, refers to combinations of two or more materials, in sheet or web form, comprising a continuous matrix phase which surrounds a reinforcing phase composed of discrete reinforcing elements. Composite materials are used in the manufacture of a wide variety of articles including automotive springs, brake pads, chemical processing equipment, air foils and fuselage structures for aircraft, helicopter blades, boat hulls, and sporting goods, such as golf clubs (shafts), baseball bats, skis and tennis rackets (frames). Such structures are commonly produced using so-called continuous fiber methods of fabrication, in which the composite material must be carefully consolidated by means of pressure and vacuum to eliminate porosity and insure complete coalescence of the matrix phase, so as to avoid non-homogeneity in the final product. These fabrication methods often require specilized equipment which adds to the capital expenditure and operating cost involved in manufacturing such structures.
Relatively recently, composite materials have reportedly been prepared from solid polymer, reinforcing material and a latex binder by an aqueous method employing conventional paper-making equipment and processes. See, for example, U.S. Pat. No. 4,426,470 which specifically describes the preparation of composite sheets using high density polyethylene as the solid polymer, glass fibers as the reinforcing material, and various latex binders of substantially water-insoluble organic polymers having either anionic or cationic bound charges. The aqueous slurries reportedly used in producing composite materials by paper-making techniques must be carefully formulated in order to obtain a satisfactory product. Special care must be taken to avoid accumulation of loose powdered polymer or resin on the surface of the paper-like web from which the composite material is prepared. The surface accumulation of loose powdered polyolefins, for example, interferes with the proper functioning of the paper-making equipment, due to the propensity of the powdered polymer to become deposited on and adhere to the heated surfaces of the drying apparatus with which the wet web or sheet comes into contact during the paper-making operation.
Special flocculants and dispersion aids have been proposed to alleviate problems caused by the surface accumulation of powdered polymer or resin. The use of such additives, however, increases the manufacturing cost of the composite material. Moreover, depending on the specific combination of components used in making the composite material, certain dispersion aids may impair the physical integrity, or otherwise adversely effect desirable characteristics of the final products. For example, when concentrated sulfuric acid is used as a dispersion aid in the production of glass fiber-containing composites, the finished mat is unusually brittle; and when a neutral dispersion aid, such as Lubrizol.TM. or Katapol.TM. is employed, and the composite material is converted to a laminated product, individual plies of the product tend to peel apart.
Another undesirable characteristic of the composite materials produced heretofore by paper-making techniques is the occurrence of "edge bleeding", which is manifested by the flow of resin beyond the periphery of the network of reinforcing elements within the body of the composite. These drawbacks have made processes for the wet lay preparation of composite material proposed heretofore, and the resultant composite material, unattractive for commercial exploitation.